Raman spectroscopy is a general-purpose detection technique in which electrons within a molecule are oscillated by an externally applied electric field from a laser, and though collisions between oscillating electrons and atomic nuclei of the molecule, the molecule begins to vibrate. Molecules in vibration are in a slightly elevated energy state compared to their ground state. The difference in energy between vibrational energy states and the ground state can be measured by determining the difference in energy between incident photons and scattered photons, a quantity known as the “Raman Shift.” Because molecules, especially complex ones, can vibrate along many different directions (called “vibrational modes”), there is not a single excited vibrational state, but rather a multitude of them, with each vibrational mode represented by a different Raman shift. Therefore, every molecule has a unique “Raman Spectrum” that may serve as a fingerprint to identify the molecule through its vibrational energy spectrum. Because the incidence of electron/nucleus collisions that result in transfer of momentum to the molecule is extremely rare, Raman scattered photons intensity is typically extremely low, and high energy lasers are required to obtain measurable Raman spectra.
Surface Enhanced Raman Spectroscopy (SERS) utilizes a metal surface to increase the interaction between the incident laser electric field and measured molecules by several orders of magnitude, thus resulting in Raman spectra that are more easily observable. Most commonly, SERS is performed on roughened metal surfaces or on the surface of metal nanoparticles, because these surfaces are capable of generating Surface Plasmon Resonance (SPR). The mechanism for SERS is not completely understood, but a common hypothesis is that electrons in the metal are excited to oscillate by the applied laser electric field (thus generating SPR), and that the metal SPR electrons can more effectively transfer their momentum to molecules that are adsorbed to the metal surface. Another hypothesis is that the surface plasmons generate locally high electric fields on the metal surface (higher than the incident laser electric field) that in turn provide greater electron oscillation within adsorbed molecules for SERS measurement.
Photonic crystals, also commonly referred to as photonic bandgap structures, are another class of sensor devices based on periodic dielectric structures exhibiting a spatially periodic variation in refractive index that forbids propagation of certain frequencies of incident electromagnetic radiation. The photonic band gap of a photonic crystal refers to the range of frequencies of electromagnetic radiation for which propagation through the structure is prevented. The photonic band gap phenomenon may be conceptualized as complete or partial reflection of incident electromagnetic radiation having selected frequencies due to interaction with the periodic structural domains of a photonic crystal. The spatial arrangement and refractive indices of these structural domains generate photonic bands gaps that inhibit propagation of electromagnetic radiation centered about a particular frequency.
Photonic crystals provide an electromagnetic analog to electron-wave behavior observed in crystals wherein electron-wave concepts, such as dispersion relations, Bloch wave functions, van Hove singularities and tunneling, having electromagnetic counterparts in photonic crystals. In semiconductor crystals, for example, an electronic band gap of energy states for which electrons are forbidden results from a periodic atomic crystalline structure. By analogy, in a photonic crystal, a photonic band gap of forbidden energies (or wavelengths/frequencies) of electromagnetic radiation results from a periodic structure of a dielectric material, where the periodicity is of a distance suitable to interact with incident electromagnetic radiation.
Selection of the physical dimensions, refractive indices and spatial distribution of structural domains of a photonic crystal provides an effective means of designing a photonic crystal a photonic band gap with a selected frequency distribution. One-dimensional, two-dimensional and three-dimensional photonic crystals have been fabricated providing complete or at least partial photonic band having selected frequency distributions gaps in one or more directions. Photonic crystals have also been fabricated having selected local disruptions (e.g., missing or differently-shaped portions of the structural domains of periodic array) in their periodic structure, thereby generating defect or cavity modes with frequencies within a forbidden bandgap of the crystal. Photonic crystals having specific defects are of particular interest because they provide optical properties useful for controlling and manipulating electromagnetic radiation, such as the ability to provide optical confinement and/or wave guiding with very little, or substantially no, radiative losses.
As diffraction and optical interference processes give rise to the photonic band gap phenomenon, the periodicity of photonic crystal structures is typically on the order of the wavelength of incident electromagnetic radiation. Accordingly, photonic crystals for controlling and manipulating visible and ultraviolet electromagnetic radiation typically comprise dielectric structures with periodic structural domains having submicron physical dimensions on the order of 100s of nanometers. A number of fabrication pathways for making periodic structures having these physical dimensions have been developed over the last decade, including micromachining and nanomachining techniques (e.g., lithographic patterning and dry/wet etching, electrochemical processing etc.), colloidal self assembly, replica molding, layer-by-layer assembly and interference lithography. Advances in these fabrication techniques have enabled fabrication of one-dimensional, two-dimensional and three-dimensional photonic crystals from a range of materials including dielectrics, metal oxides, polymers and colloidal materials.
Because PC sensors are comprised of dielectric materials, they will not quench fluorophores within <30 nm of their surface by resonant energy transfer, and they can exhibit high Q-factors due to their low absorption loss. One embodiment is comprised of a one-dimensional (1D) or two-dimensional (2D) periodic surface structure formed from a low refractive index (RI) dielectric material that is overcoated with a high RI thin film, these devices can be fabricated upon plastic substrates inexpensively over large areas by a nanoreplica molding process and incorporated into the surface of glass slides, microfluidic channels, and microtiter plates. The device period, grating depth, film thicknesses, and RIs of the materials are chosen in such a way that the PCs can support guided-mode resonances at designated wavelengths, where the device reflects ˜100% of incident light at the resonant wavelengths with all other wavelengths being transmitted. Under resonant conditions, excited leaky modes are localized in space during their finite lifetimes, which enhances the near electric-field intensity of the PC structure and thus enhances the excitation of fluorophores near the PC surface.
Further background information relating to photonic crystals sensors and their properties and methods of manufacture are disclosed in the following references, which are incorporated by reference herein: P. C. Mathias, N. Ganesh, L. L. Chan, and B. T. Cunningham, “Combined enhanced fluorescence and label-free biomolecular detection with a photonic crystal surface,” Applied Optics, vol. 46, pp. 2351-2360, 2007; N. Ganesh, W. Zhang, P. C. Mathias, E. Chow, J. A. N. T. Soares, V. Malyarchuk, A. D. Smith, and B. T. Cunningham, “Enhanced fluorescence emission from quantum dots on a photonic crystal surface,” Nature Nanotechnology, vol. 2, pp. 515-520, 2007; N. Ganesh and B. T. Cunningham, “Photonic-crystal near-ultraviolet reflectance filters fabricated by nanoreplica molding,” Applied Physics Letters, vol. 88, 2006; C. J. Choi and B. T. Cunningham, “Single-step fabrication and characterization of photonic crystal biosensors with polymer microfluidic channels,” Lab on a Chip, vol. 6, pp. 1373-1380, 2006; B. T. Cunningham, P. Li, S. Schulz, B. Lin, C. Baird, J. Gerstenmaier, C. Genick, F. Wang, E. Fine, and L. Laing, “Label-free assays on the BIND system,” Journal of Biomolecular Screening, vol. 9, pp. 481-490, 2004; S. S. Wang, R. Magnusson, J. S. Bagby, and M. G. Moharam, “Guided-Mode Resonances in Planar Dielectric-Layer Diffraction Gratings,” Journal of the Optical Society of America α-Optics Image Science and Vision, vol. 7, pp. 1470-1474, 1990; R. Magnusson and S. S. Wang, “New Principle for Optical Filters,” Applied Physics Letters, vol. 61, pp. 1022-1024, 1992; S. S. Wang and R. Magnusson, “Theory and Applications of Guided-Mode Resonance Filters,” Applied Optics, vol. 32, pp. 2606-2613, 199; C. Y. Wei, S. J. Liu, D. G. Deng, J., Shen, J. D. Shao, and Z. X. Fan, “Electric field enhancement in guided-mode resonance filters,” Optics Letters, vol. 31, pp. 1223-1225, 2006.
Biosensors incorporating photonic crystal structures are described in the following references which are hereby incorporated by reference in their entireties: U.S. Pat. Nos. 7,118,710, 7,094,595, and 6,990,259; U.S. Published applications 2007/0009968; 2002/0127565; 2003/0059855; 2007/0009380; 2003/0027327; and Cunningham, B. T., P. Li, B. Lin and J. Pepper, Colorimetric Resonant Reflection as a Direct Biochemical Assay Technique, Sensor and Actuators B, 2002, 81, pgs 316-328; and Cunningham, B. T. J. Qiu, P. Li, J. Pepper and B. Hugh, A Plastic Calorimetric Resonant Optical Biosensor for Multiparallel Detection of Label Free Biochemical Interactions, Sensors and Actuators B, 2002, v. 85, pgs 219-226.